Table of contents

Appendices

4.7 Abstract Elements and Types

ref26XML Schema provides a mechanism to force substitution for a particular element or type. When an element or type is declared to be "abstract", it cannot be used in an instance document. When an element is declared to be abstract, a member of that element's substitution group must appear in the instance document. When an element's corresponding type definition is declared as abstract, all instances of that element must use xsi:type to indicate a derived type that is not abstract.

In the substitution group example we described in [Substitution Groups], it would be useful to specifically disallow use of the comment element so that instances must make use of the customerComment and shipComment elements. To declare the comment element abstract, we modify its original declaration in the international purchase order schema, ipo.xsd, as follows:

NOTE: 

<element name="comment" type="string" abstract="true"/>

With comment declared as abstract, instances of international purchase orders are now only valid if they contain customerComment and shipComment elements.

Declaring an element as abstract requires the use of a substitution group. Declaring a type as abstract simply requires the use of a type derived from it (and identified by the xsi:type attribute) in the instance document. Consider the following schema definition:

NOTE: 

Schema for Vehicles

<schema xmlns="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://cars.example.com/schema"
           xmlns:target="http://cars.example.com/schema">

  <complexType name="Vehicle" abstract="true"/>

  <complexType name="Car">
    <complexContent>
      <extension base="target:Vehicle"/>
    </complexContent>
  </complexType>

  <complexType name="Plane">
    <complexContent>
      <extension base="target:Vehicle"/>
    </complexContent>
  </complexType>

  <element name="transport" type="target:Vehicle"/>
</schema>

The transport element is not abstract, therefore it can appear in instance documents. However, because its type definition is abstract, it may never appear in an instance document without an xsi:type attribute that refers to a derived type. That means the following is not schema-valid:

NOTE: 

<transport xmlns="http://cars.example.com/schema"/>

because the transport element's type is abstract. However, the following is schema-valid:

NOTE: 

<transport xmlns="http://cars.example.com/schema"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:type="Car"/>

because it uses a non-abstract type that is substitutable for Vehicle.